FIG. 1 illustrates an architecture of a mobile communication system.
Referring to FIG. 1, an LTE User Equipment (UE) 105 is located in an area where the Mobility Management Entity (MME) 110 area including an MME 115 and an Evolved Universal Mobile Telecommunication System Terrestrial Radio Access Network (E-UTRAN) 120 and an Wideband Code Division Multiple Access (WCDMA) area 125 including a UTRAN 130 and a Mobile Switching Center (MSC) 135 are overlapped. The UE 105 is capable of communicating with the E-UTRAN 120 and UTRAN 130.
In order for the LTE UE 105 located in the LTE network to handle a Circuit-Switched (CS) call process, it is required to operate with CS Fall Back (CSFB) function. That is, once the network switching from the LTE network to the WCDMA network 125 has completed using the CSFB function, the UE 105 can place an outbound CS call or receive an inbound CS call. Once the CS call has been processed completed, the LTE UE 105 is capable of returns to the LTE mode according to a switching policy.
In order to achieve this, the LTE UE 105 has to follow the handling rules between E-UTRAN 120 and UTRAN 130 and between MME 115 and MSC 135.